Residuum coking and cracking



Oct. 13, i953 J. w. BROWN ETAL RESIDUUM COKING AND CRACKING 5Shets-Sheet l Filed June 9. 1951 FLUE GAS To FQACUQNATQL #IGM-i Of- 13,1953 J. w. BROWN ETAL RESIDUUM COKING AND CRACKING l 2,655,464 FiledJune 9. 1951 3 Sheets-Sheet 2 FLue @A5 E To Drzonucr 22 g 12mm/:lv 121u4- 112:

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f iOiGL f STnAM T-T I G 2 es E. Ua agg gn-venborfs Oct. 13, 1953 J. w.BROWN ETAL RESIDUUM COKING AND CRACKING 2,655,464 Filed June 9. 1951 3Sheets-Sheet 3 CATALYST QE GE M E QATOQ 181 [2E AcToQ E Patented oci.13, p 1953 2 655 464 RESIDUUM CGKING AND CRACKING James W. Brown,Elizabeth, and Charles E. Jahnig, Red Bank, N. J., assignors to StandardOil Development Company, a corporation of Delaware Application June 9,1951, Serial No. 230,746 5 Claims. Cl. 196`49) is primarily accomplishedthereafter in a catalo tamination, and a maximum yield of high quallyticmanner. Specifically, the vapors resulting ity motor fuel fractions.Other objects will be from the coking zone are passed to a crackingapparent from the subsequent description of the carbonaceous deposit. Acharacteristic feature a heavy hydrocarbon feed is contacted with hotthe coking zone as well as the catalytic cracking bed, followed by acracking step wherein the zone are supplied by direct mixing of hotreresulting hydrocarbon vapors are contacted with generated catalystwith the inert solids 1n a mixa dense fluid bed of a cracking catalysthaving ing zone, whereupon catalyst and reheatedinerts a particle sizedistribution substantially differ- However, such processes have not been`cornmeroff while in a dense iluid phase, a heat exchange ciallyattractive in view of the great heat restep wherein the hot regeneratedcatalyst is quirements involved, especially since it has been mixed withcoke particles withdrawn from the a feed cut which had to be revaporlzedin the ation or the like into its two principal compofeeding of a'heatcarrying medium such as coke 35 ing of the seed coke withdrawn from thecoking similarly Valuable extraneous fuels such as fuel 40 illustratedby severalspeciiic examples wherein gas. In still other known fluidcoking processes. reference is made` to the accompanying drawing. cokeand a cracking catalyst or inert inorganic Fig. 1 is a semi-diagrammaticillustration of to mixing with hot regenerated catalyst in a separatemixing and elutriation zone whence reheated coke particles are returnedto the cokng Zones and regenerated catalyst is returned to the catalyticcracking zone;

Fig. 2 illustrates an alternative modification of the invention,according to which the catalytic cracking zone is superimposed on thecoking zone in the same vessel which also contains themixing-and-elutriation Zone in direct communication with the cokingzone; and finally Fig. 3 illustrates a third alternative according towhich low temperature coking oi the hydrocarbon feed is essentiallycompleted in a transfer line wherein the feed is mixed with hot cokeparticles prior to admission to the catalytic cracking acne wherefromrelatively fine catalyst is entrai ed overhead with the product vaporseventual recycling to the cracking zone whereas the relatively coarsecoke is withdrawn at the bottom and mixed with further amounts ofhydrocarbon feed.

Referring now in detail to Fig. l of the drawing, reduced crude such asan 8% bottoms fraction of about lil-30 Conradson carbon, oi about 5 APgravity and obtained from the vacuum distillation of a West Texas crude'or a similar heavy residue is sup-plied to coking Zone 2 through line ias a liquid at a temperature of about 300 to 800 F., or preferably atabout 700 lneit solids, preferably petroleum coke particles having aparticle size in the range between about 100 and 500 microns aremaintained in the cokf 35 coking zone, e. g. at 1000 F., but asubstantial ing Zone at a temperature of about 850 F. to 950 F. as adense, turbulent, uidized bed 3 having an upper level d and an apparentdensity of about l to 50 lbs/cu ft. with densities of about 0.01 tolbs/cu. ft. above level 4, whileAV an inert gas such as steam introducedthrough lines 30, 3| and 32 as later described is being passed upwardlythrough the coke particles at a linear superficial velocity of about 0.5to 5 feet per second.

The vapors produced in the hot coking Zone 5 2. are passed therefromthrough perforated plate 5 into the superimposed catalytic cracking Zone5 maintained at about 900 F. to 1000D F. and containing a dense,turbulent, fluidized bed 1 of a cracking catalyst such as one of theknown synthetic silica-alumina composites. The physical characteristicsof catalyst bed 1 are essentially similar to those of the coke bed 3described above, except that the catalyst particle size is within therange of about 50 to 150 microns and is preferably at least to 50microns smaller than the smallest particles constituting a substantialportion of the coke in bed 3, so as to assure an efficient separation ofthe two solids in the subsequent elutriation. Thus, for instance, whencoke ranging in size down to about 100 microns is used in the coker, thecatalyst particles are preferably in the range between 20 and 50 or 80microns, but when the coke particles of the process range in sizebetween about 365 200 and 300 microns, catalyst particles in the rangeup to about 150 microns may be used. The cracked. hydrocarbon vapors arewithdrawn from cracking zone Ei through cyclone 8 or other gas- I solidseparator means and passed through line 0 to a conventional finishingsystem for recovery of naphtha, gas oil and other desired hydrocarbonfractions.

Spent catalyst is withdrawn from bed 1 through standpipe l0, which mayhave taps ll 7 for admitting a small amount ci an aeration gas, and thewithdrawn catalyst is finally mixed with air admitted through line l2and passed to regenerator I3 where carbonaceous deposits are 5 burnedoff the catalyst in fluid phase at a temperature of about 1100* F. tol250 F. in a manner well known per se. Excess heat may be removed fromthe system by means of heat exchanger l4 and hot regenerated catalyst isuse l0 to supply the heat requirements of the coking zones as well asthe catalytic cracking zones in the manner described later herein.Moreover, especially where exchanger Hi is intended for preheating feedand thus supplying a portion l5 of the regeneration heat directly to thecoker, it

may be necessary to locate the exchanger in a separate vessel throughwhich hot catalyst is circulated. This allows greater flexibility andcontrol over regeneration temperature without 20 causing undue coking offeed within the heat exchanger, as might otherwise be the case ifregenerator temperature permitted only a low feed circulation ratethrough the exchanger when immersed directly in the regenerator bed asshown in Fig. l.

Coke from bed 3 of the coking zone is withdrawn through downcomer l5 toanother stage It where the coke is again maintained as a dense,turbulent, fluidized bed ll and where any unconverted, oily residuumwhich may be adhering to some of the coke particles is cracked andconverted into vapors and dry coke. The second and any furtherconsecutive stages are preierably at a higher temperature than the mainadvantage is obtained even when all stages are at the same temperature'as the main coking Zone 2, since the main purpose of such staging isthe prevention of oil soaked, incompletely 40 coked particles leavingthe coker bed in the coke portion withdrawn to the mixer as laterdescribed, which portion is a fairly representative sample of theparticle mixture present in the particular bed.

For example, where only a one-stage coker is used, 22% of the cokewithdrawn therefrom will have a residence time equal to less vthan 25%of the average or nominal holdup time, and such particles usuallyexhibit a considerable degree of stickiness. But, by using a 2-stage oreven a 3-stage coker, the percentage of particles being withdrawn fromthe last stage having a residence time equal to less than 25% of theaverage total holdup Vis reduced drastically to l0 and 4%, respectively,and consequently the stickiness of the resulting coke product ismaterially decreased.

Net coke product may be withdrawn from the last bed l1 through pipe I8,it being particularly desirable to remove those coke particles whichhave grown too large for good iiuidiaation in the system. Control of thecoke particle size in the system can be achieved by screening out thelargest coke particles while recycling the smaller ones, or thewithdrawn particles may even be ground before being returned to thesystem. However, where grinding is employed, it is desirable to removealso the fines from the coke to be returned, as otherwise such cokefines would eventually become mixed with the catalyst and undesirablyincrease the necessary amount of carbon burning capacity.

A side stream of the coke from the dry coke bed l1 is withdrawn throughline I8 to mixer- '5 elutriator 20 where the coke particles are mixedrwith hot catalyst particles withdrawn from remildest crackingconditions. After passage generator I3V through line 2| while an inertgas through coke bed |01 the hydrocarbon vapors such as steam isintroduced into the bottom of liberated in the coking steps passoverhead from they exhange heat so that the catalyst is cooled below thecracking section |09 and next to coking and the coke heated to about1100 F'. and more- 10 section |05.

over, due to the previously described difference in In the crackingsection the hydrocarbon vapors particle size, the relatively finecatalyst is stripped flow up through the dense, turbulent. iuidized bedfrom the Iiuidized mixture in vessel 20 and enof cracking catalyst |2 atabout 900 F.1000 F'.

lytic conversion step. Spent catalyst is stripped with steam in zone |28Coarse coke particles, essentially free of catalyst in the usual mannerand goes through line I to and reheated by contact with the regeneratede regenerator ||6 where the carbonaceous dethrough standpipe 23 andafter admixing with 20 combustion. Air is bloivn into the regenerator ofthe hot mixing vessel V and standpipe 24 30 opening |24 to thesegregated mixingsectionlHin her bed |1 and pipe I8 described earlierherein S eam Withdrawn from the coker bed |01 through Referring to Fig 2the modification illustrated .Wall Opening |25 Which is located at alevel intel ereln accomphshes the purposes 0f the lnvenmediate betweenthe Catalyst 111181; |24 and the zone is directly in the main reactorand conseinjected thTOllgh line |29 at the bottom of the quentlyrequires a, high pressure-drop grid bemixing Section at a rate to giVean upward gas tween the coking zone and the superimposed cat- Velocityin the mixing section between about o 1 in the Conversion Z0netrainedupward from the catalyst-coke mixture In Operating this system, theheavy hydrowhile the coke particles are uidized but are not carbon feedpreheated to about 700 F. is intro- Substamlally entraned overhead.

duced through line |0| and heated to about 800 The regenerated Catalystis thus carried into to 1100 11". by mixing With colse particles WiththeCracking SCOD |09 Where it again becomes line |0|a is then passedupwardly through transthe Ffgellelatr- This facilitates Operating thefer line |04 at a rate of about 15 to 50 feet per 55 Crackbmg react@ atH1 temperature IOWeI than second, equivalent to a transfer lineresidence that 1I1 the C0k1ng Zone. The relatively coarse with only aminimum formation of thermally System can be Obtained by elutration,Screening, cracked naphtha. After this first-stage coking iii vSelectiveCyGlOne action 0r the like. For instance, the transfer line, colii'ng ofthe feed is completed `this can be done by locating a cyclone |26 insoine in the Coker bed |01 Where any eolie particles 0r even in allOpenings of the reactor grid |10 containing a surface Iilm ofincoinpletely coked and by withdrawing the separated coke nes from fromthe system at least periodically, or with the product coke.

Likewise, where catalyst particles tend to stick to the coke in the heattransfer mixer so that they would be eventually rejected from the systemalong with the product coke, further modification of the system may bedesirable. For example, the cufculty can be overcome by mixing the cokeas it leaves the coking bed. with hot recycle coke and then providingsufficient holding time to dry the coke in an intermediate zone beforeit contacts the hot catalyst in the heat transfer zone and strippingsteam can be added to assist the operation.

Referring back to grid which supports the catalyst bed in the crackingsection |09 above the dilute phase of the coking section |05, it isimportant that the pressure drop across the grid be such as to cause thedesired upward flow of catalyst from the mixer-elutriator iii to thecatalyst bed ||2 while preventing fiow of catalyst to the coker bed |01throughwall opening |25. The required pressure drop will usually rangebetween about 1 and 5 or 10 pounds per square inch and must be at leastenough to offset the differential between the hydrostatic pressureexerted by the relatively dense upward stream of catalyst finessuspended in steam in the upper portion of the mixer-elutriator sectionof the reactor vessel the apparent density of this phase being betweenabout 10 and i0 lbs. per cu. ft., and, on the other hand, the adjacentdilute phase existing above level |08 of the fluid coker bed |101. Ingeneral, the velocity through the openings in grid l l0 should bemaintained in the proper range to give the required pressure drop so asto coinpensate for the lower density in the dilute phase above level |08compared to that in the upper part of zone In the operation of thissystem, the temperature at the coke-catalyst mix point is regulated togive the desired heat balance. The coke circulation rate is controlledby the valve in line |02 to supply the desired heat to coking zone |05,and the catalyst rate will be fixed by the heat released in theregenerator and the difference between the temperatures in lines H5 and|23, although auxiliary cooling (or heating) means may be provided.Also, additional flexibility is possible by iowing part of the catalystthrough bypass line |30. These controls allow operation over a widerange of coker, cracking, and regeneration temperatures, and provideflexibility in catalyst/ oil ratio.

Fig. 3 illustrates still another embodiment of the invention. Inoperating this system a liquid residuum of the type previouslydescribed, preheated to about 300 to 800 F. may be supplied through line|42. Hot inert solids, specifically coke having a particle size of about100 to 500 microns may be added from standpipe his at a temperature ofabout 900 to 1100 F. in an amount of about 700 to '7,000 lbs/barrel. Bythis addition of hot coke the temperature of the feed may be raised toabout 800 to 1000 F. whereby the more volatile components of the feedare more or less completely vaporlzed without, however, being convertedto naphtha and lighter products in any substantial degree. The lessvolatile components of the feed are deposited on the coke particlesduring their travel through line |43 and before they enter the reactor|40. In particular it is desirable to make line |43 sufciently long toallow for a residence time therein of about 0.5 to 5 seconds so thatsubstantially all inorganic salts'and other contaminants as Well as alarge proportion of carbon from the asphaltenes `and other carbonforming constituents of the feed are deposited on the coke particles andthe resulting carbonaceous deposit is sufficiently dried to avoid unduestickiness of the coke particles after they enter reactor |46.

The resulting dispersion of coke in hydrocarbons may enter reactor |46through distributing grid |68 to form above the grid a dense turbulentfluidized bed or mass M146 of the type specified earlier with referenceto the reactors of Figures 1 and 2. About r100 to 5,000 lbs. of hotregenerated catalyst per barrel of feed may be supplied f from standpipe|50 as will appear hereafter. As

in the previously described examples, this catalyst has a particledistribution range below the range of the above-described cokeparticles, for instance, the catalyst may have a particle size up toabout microns and must be readily entrainable at the fluidizationconditions of reactor |46. Mass M146 is maintained at a temperature ofabout 900 to 1l00 F. conducive to the desired catalytic crackingoperation and may be composed of a highly active silica-aluminacomposite or other known cracking catalyst.

Reactor |46 may be provided with an elutriation well |52 into the bottomof which an elutriation gas such as steam or a light hydrocarbon gas isadmitted via line |54. Preferably the elutriation well is asmall-diameter vertical section which extends downwardly from a pointjust below dense bed level Lus so as to be in open communication withdense, fluidized bed M146 and is preferably lled with a packing ofbodies of non-fluidizable size, such as Raschig rings or the like,having interstices which permit percolation of the fluidized solidswithin the packing. The elutriation gas may be supplied via line |54 ata rate sufficient to elutriate from packed section |52 substantially allthe catalyst so that coke particles practically free of catalyst may bewithdrawn via standpipe |-4i and supplied to line |43 in the manner andfor the purpose described above. Any desired portion of the withdrawncoke may be drawn off standpipe ldd via line |50 for the removal of netproduct coke, together with the contaminants deposited thereon. Purifiedor make-up solids may be supplied to the system via line |50, as forexample seed coke, or make-up catalyst.

The catalyst which may contain up to about 3 wt. per cent of coke isreadily entrained by the product vapors and elutriation gas and carriedin the form of a dilute suspension overhead from mass Mms through line|00 into separator |62 from which product vapors may be passed torecovery via line |04. Separated catalyst, preferably stripped ofhydrocarbons by injection of an inert gas such as steam through one ormore taps t to a stripping zone may be supplied via line |56 to a lowerportion of catalyst bed M168 in regenerator |65. Simultaneously, air isblown into regenerator |08 through line |'0 and grid |12 at conditionssuitable to maintain the catalyst mass Misa in a dense, turbulentcondition at a temperature of about 1000 to 1280o F. by combustion ofcoke deposits in a manner well known per se and heretofore outlined inconnection with the catalyst regenerators of Figs. 1 and 2. Flue gasesand entrained catalyst may pass into cyclone |14 from which separatedcatalyst fines may be returned via dip leg |153 or discarded via line|18, flue gases being withdrawn via line |80. Make-up catalyst may beadded aanmet- Q Having described specific embodiments of the ble to thetreatment of heavy residual crude reactor temperatures, as otherwise theWet coke be reduced -crudesobtamed hy atmospheric or andai-'1ct coke`particles are mixed therein in a.

to tar from visbreaki'ng operations and to other 20 by mixing with about0.2 to 2 parts of hot regencles in Vapor, as lle 3| 0f Fg- 15 1m@ |04 40ereneto cokerxsoldsthugh-it wiilbe underperse phases above the densebeds'. In elutriation zones such as zone of` Fig. '1, zone `H I `o1' 50sql'dsas'hretofo described; va

on the particle size and density, @as -well as lsize 105s fbrtlyt' 'ugtosize from aboutrlOO to i300 or -500 microns,

Moreover ,partculaylm startmlgup the open ,eneration in a manner obviousto those smiled as hereafter `described at somewhat greater lThe inertsonas are heldlup mule primary 75 `catalyst regenerator such. as Vesseli168 and inally 11 returned therefrom Vthrough standpipe 150 t0 thereactor.

Reaction conditions may include coking ternabout 800 to 1200 F.,catalytic cracking temperatures of about 800 to 1000 F. and catalystregeneration temperatures of about 1000 to 1200 or 1300 F., depending onthe nature of the catalyst used. Oi course, the regenerator temperatureand the rate of circulation of catalyst and coke to the inert solids arereheated to the temperature by direct heat exchange lyst beforeseparation and recyclingI version zones, are so adjusted as to tendedtemperature conditions both in the zones and the catalytic crackingzones.

Having given a full description of the invention and of the manner ofusing it, the invention is particularly pointed out and distinctlyclaimed in the appended claims.

We claim:

1. A process for convertng a residual petroleum feed stock boilingpredominantly above 900 F. into ts and coke which lighter produccomprises introducing the residual stock into a primary coking zone tothe congive the incoking wherein coke particles of a size between 100and 500 microns are maintained at a temperature between about 800 and1100 F. as a dense fluidized bed with a less dense phase thereabove, andwherein the petroleum stock is vaporized and partially coked substantialconversion to naphtha and lighter products, passing the resultingpetroleum vapors upwardly through a cracking zone maintained at about900 to 1100 F. and containing a dense luidized bed of cracking catalystranging in particle size up 100 microns with a less dense phase abovesaid bed, recovering cracked vapor products from the cracking zone,passing spent catalyst from the cracking zone to a regeneration zonemaintained at a temperature between about 1000 and 1300 F. through whichan oxygen-containing gas is passed in an upward direction at a ratesufficient to maintain the catalyst particles as a dense uidized masswhile burning ofi deposited carbon, overflowing the coke particlescontaininga wet coat of partially coked feed from the aforesaid primarycoking zone to at least one secondary coking zone wherein thecokeparticles are maintained at a tem- `perature above 800 F. as a denseluidized mass by upward passage of a dry inert gas therethrough,recovering dry net coke product from the secondary coking zone, passinga portion of the dried coke from the secondary coking zone to a mixingzone, also passing a portion of catalyst at a temperature above 1100 F.from the regeneration zone to the mixing zone and there intimatelymixing it with the dried coke, passing an inert gas upwardly through theresulting particle mixture in at the minimum fluidization rate of thecoke and tively returning the entrained catalyst particles from themixing zone to the conversion process, withdrawing catalyst-iree,reheated coke particles from the bottom of the mixing zone and returningthe reheated coke to the primary coking zone. 2. A process according toclaim 1 wherein the secondary coking zone, the primary coking zone andthe catalytic cracking rone are superimposed above each other in thesequence recited.

3. A process according to claim 1 wherein some coke withdrawn from the'mixing zone is returned to each of the coking zones and wherein thesecondary coking zone is maintained at a temperature' between 1000 and1200 F. and substantially higher than the temperature oi the primarycoking zone.

4. A process for converting a residual hydrocarbon feed characterized bya Conradson carbon value in excess of 5 which comprises preheating thefeed to a temperature between 600 and 800 F., mixing the feed with cokeparticles ranging in size between about and 300 microns and heated to atemperature between about 800 and 1200 F. to form a dilute suspension ofcoke in hydrocarbon vapors, passing the resulting suspension at atemperature oi about 800 to 1100 F. through an elongated and constrictedprimary coking zone at a superficial gas velocity of about 5 to 50 feetper second to a secondary coking zone of wider diameter wherein theupward gas velocity is within the range between 0.5 and 4 feet persecond and wherein the suspended coke particles are accordinglymaintained as a dense, turbulent, uidized mass, withdrawing theresulting hydrocarbon vapors from the secondary coking zone into andupwardly through a catalytic cracking zone superimposed thereabove,interposing a pressure drop of about to 5 lbs/sq. in. in the path of thehydrocarbon vapors between the secondary coking zone and the crackingzone, contacting the hydrocarbon vapors with catalyst particles rangingin about 0 and 80 microns and a dense turbulent fluidized mass in thelower portion of the cracking zone at a temperature of about 900 to 1100F., withdrawing cracked hydrocarbon vapors overhead from the crackingzone, withdrawing spent catalyst from the dense turbulent mass of thecracking zone and passing it to a regeneration zone where it ismaintained as a dense, turbulent, fluidized mass and regenerated byupward passage of an oxygen-containing gas therethrough at a temperaturebetween 1000 and 1200 F., withdrawing hot catalyst from the regenerationzone and passing it to a lower portion of a mixing zone located adjacentto the secondary coking zone and below the cracking zone, overowing cokeparticles from the dense turbulent mass oi the secondary coking zoneinto an upper portion oi the mixing zone where the coke becomes reheatedby countercurrent contact with the regenerated catalyst, introducing aninert gas near the bottom of the mixing zone at a rate sufficient toentrain the catalyst particles from the catalyst-coke mixture into thesuperimposed cracking zone, withdrawing reheated coke particles from thebottom portion of the mixing zone, and mixing the withdrawn coke withincoming iced.

5. A process for converting heavy hydrocarbons which comprises mixing aheavy hydrocarbon feed with coke particles ranging in size between about100 and 300 microns and heated to about 900 to 1200 F. to form a dilutesuspension of oilcoated coke particles in hydrocarbon vapors, passingthe resulting suspension through a constricted elongated coking zone ata temperature between 800 and 1100 F. at a rate of 5 to 50 feet persecond for a time of about 0.5 to 5 seconds and thereby coking the oilycoat deposited on the coke particles, thereafter introducing thesuspension into a cracking zone oi relatively large cross-sectionwherein the upward velocity of the hydrocarbon vapors is reduced toabout 0.5 to 5 feet per second and wherein the introduced suspensionbecomes `mixed with cracking catalyst oi the reheated n vapors overheadfrom the cracking zone. 15

JAMES W. BROWN. CHARLES E. JAHINIG.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS FOR CONVERTING A RESIDUAL PETROLEUM FEED STOCK BOILINGPREDOMINANTLY ABOVE 900* F. INTO LIGHTER PRODUCTS AND COKE WHICHCOMPRISES INTRODUCING THE RESIDUAL STOCK INTO A PRIMARY COKING ZONEWHEREIN COKE PARTICLES OF A SIZE BETWEEN 100 AND 500 MICRONS AREMAINTAINED AT A TEMPERATURE BETWEEN ABOUT 800 AND 1100* F. AS A DENSEFLUIDIZED BED WITH A LESS DENSE PHASE THEREABOVE, AND WHEREIN THEPETROLEUM STOCK IS VAPORIZED AND PARTIALLY COKED WITHOUT SUBSTANTIALCONVERSION TO NAPHTHA AND LIGHTER PRODUCTS, PASSING THE RESULTINGPETROLEUM VAPORS UPWARDLY THROUGH A CRACKING ZONE MAINTAINED AT ABOUT900 TO 1100* F. AND CONTAINING A DENSE FLUIDIZED BED OF CRACKINGCATALYST RANGING IN PARTICLE SIZE UP TO ABOUT 100 MICRONS WITH A LESSDENSE PHASE ABOVE SAID BED, RECOVERING CRACKED VAPOR PRODUCTS FROM THECRACKING ZONE, PASSING SPENT CATALYST FROM THE CRACKING ZONE TO AREGENERATION ZONE MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 1000 AND1300* F. THROUGH WHICH AN OXYGEN-CONTAINING GAS IS PASSED IN AN UPWARDDIRECTION AT A RATE SUFFICIENT TO MAINTAIN THE CATALYST PARTICLES AS ADENSE FLUIDIZED MASS WHILE BURNING OFF DEPOSITED CARBON, OVERFLOWING THECOKE PARTICLES CONTAINING A WET COAT OF PARTIALLY COKED FEED FROM THEAFORESAID PRIMARY COKING ZONE TO AT LEAST ONE SECONDARY COKING ZONEWHERE-